1. Field of the Invention
The present invention relates to colouring substance-containing liquid crystal mixtures, their manufacture and their use in electro-optical devices.
2. Background Description
Liquid crystalline mixtures which consist of a liquid crystal matrix (host) and dichroic colouring substances (guest) have a different light absorption depending on orientation. This effect is known in the literature as a guest-host effect and is used in electro-optical indicating devices. Further, it is known that the contrast can be improved by adding an optically active material to such mixtures.
The guest-host cell (guest-host display) comprises essentially a condenser, at least one electrode being transparent and the dielectric being formed from a nematic or cholesteric liquid crystal which contains one or more dichroic colouring substances. Since the usable colouring substances mainly have positive dichroism, i.e. the transition moment of the absorption of visible light lies approximately in the direction of the longitudinal molecular axis, the homogeneous orientation of the liquid crystal (longitudinal molecular axis parallel to the surface of the plates) generally corresponds to the coloured condition of the cell and the homeotropic orientation (longitudinal molecular axes perpendicular to the surface of the plates) generally corresponds to the colourless condition of the cell. For example, a liquid crystal with positive dielectric anisotropy (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &gt;O, .epsilon..sub..parallel. signifying the dielectric constant along the longitudinal molecular axis and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto), which is homogeneously oriented by treatment of the surface of the plates, is orientated homeotropically by the application of a voltage. In this manner colourless symbols can be shown on a coloured or black background ("negative" contrast). On the other hand, a liquid crystal with negative dielectric anisotropy, which is homeotropically orientated by treatment of the surface of the plates, is orientated homogeneously by the application of a voltage, whereby the reading of coloured or black image elements on a colourless background is made possible ("positive" contrast).
The customary, static operation of liquid crystal indicating devices has in the past been replaced to an increasing extent by the so-called multiplex control. In general only multiplex ratios of about 1:8 to 1:10 have been attained employing the usual amplitude-selective multiplex procedures.
However, in order to improve the multiplex ratio in the multiplex control of liquid crystal indicators, especially of rotation cells and guest-host cells, a two-frequency matrix addressing procedure has been proposed (e.g. German Offenlegunsschriften Nos. 2 856 134 (Great Britain Pat. No. 2 013 104) and 2 907 940 (Great Britain Pat. No. 2 020 075. This makes use of the fact that the dielectric anisotropy of liquid crystals, which upon application of a low-frequency voltage have a positive anisotropy of the dielectric constants, is negative in the case of high frequencies. Not only positive but also negative contrasts can be produced with this procedure. Further, the two-frequency procedure offers the advantage that not only the switching-on operation, but also the switching-off operation can be influenced directly by the application of a corresponding alternating voltage, whereby an acceleration of the switching-off operation is achieved.
The previously known colouring substance-containing liquid crystal mixtures have above all the disadvantage that either the solubility or the degree of order of the colouring substance is too small or the stability towards UV-radiation is insufficient. Further, the previously known mixtures generally have relatively long response times.
The object of the present invention is therefore to provide novel coloured mixtures which do not have the aforementioned disadvantages or which have these disadvantages to lesser extent.